1. Field of the Invention
This invention relates to a rotating head magnetic tape drive. More particularly, the invention relates to method and apparatus for dynamically correcting skew error between the transverse track on the magnetic tape and the path of the rotating head.
2. Discussion of Prior Art
The problem of skew error correction in cross tape magnetic recording is quite old. It commonly arises with regard to two environments. First, static adjustments have been made to bring the tape into proper alignment with the rotating head and the mandrel that supports the tape along the path of the rotating head. An example of such a teaching in U.S. Pat. No. 3,697,676, entitled "Head to Tape Alignment Apparatus and Method," invented by Arthur I. Protas.
The Protas patent teaches that parallelism between the tracks on the tape and the path of the rotating head may be adjusted in a static manner by moving either an entry or exit post guide that guides the tape onto or off of the rotating-head mandrel. Once the parallelism between the path of the magnetic head and track is statically adjusted, moving the parallel track into alignment with the head is accomplished by moving both guides laterally relative to the rotating-head mandrel. Static adjustment of the guide, as herein referred to, means that the guide is moved once during a long period of time manually by an operator. This is in contrast to a dynamic skew error correction which would be accomplished by an automatic system moving edge guides continuously or repeatedly over a short time interval to maintain parallelism between the track and the path of the rotating head.
Another rotating-head recording environment in which it is desirable to change the angle of the track on a tape to bring it into parallelism with the path of the rotating head is in converting from a stop mode to a slew mode operation. In stop mode recording operation in a rotating head magnetic recorder the tape is held stationary as the rotating head sweeps a single time or repeatedly over a single track. In slew mode the tape moves continuously and is synchronized in speed with the rotational speed of the head so as to control the angle that the path of the head makes with the longitudinal dimension of the tape. Clearly, to switch from a stop mode to a slew mode of operation would require a change in the angle of the track on the tape to maintain parallelism between the track and the path of the rotating head.
Apparatus for adjusting tape path to handle this type of track angle realignment is taught in U.S. Pat. No. 3,376,395, issued to W. L. Rumple. The Rumple patent shows crowned pulleys to guide the tape onto the rotating-head mandrel. The mandrel itself has fixed guides mounted thereon and is adjustable laterally and statically to change the separation of the fixed guides. When the Rumple magnetic recorder is switched to a stop mode (tape stationary during read or write) operation, the fixed mandrel guides are moved farther apart. The crown pulley entry and exit guides then cause a shift in tape angle across the mandrel as the tape moves between the fixed guides spaced farther apart. The shift in tape angle across mandrel changes the track angle relative to the path of the rotating head. The adjustment in the Rumple patent is a manual adjustment by an operator at the time the conversion between stop and slew mode is made.
Dynamic adjustment of entry or exit guides for tape helically wrapping a mandrel has not been done in the past because it was well accepted in the art that to do so would distort the recorded track. The reason for the distortion is the geometry of the tape path relative to the rotating-head mandrel. This geometry controls whether there is equal or unequal tension across the width of the tape as the tape wraps the mandrel. For example, if the tape helically wraps the mandrel 360.degree., the angle at which the mandrel should be positioned relative to the longitudinal dimension of the tape is given by the expression sin .theta. = G.sub.s /C where G.sub.s is the guide spacing (distance between the same edge on the entry and exit guides) and C is the circumference of the mandrel. This angle also corresponds to the acute angle of a track written by the rotating head relative to the edge of the magnetic tape.
It was well accepted in the art that if the guide spacing were to change without changing the angle .theta. between the mandrel and the tape, an unequal tension distribution would exist across the width of the tape. This unequal tension in the tape would cause the tape to distort and consequently, a written track on the tape would distort relative to the path of the rotating head. This expected distortion will be discussed further hereinafter.
The problem of distortion is heightened by the use of wide magnetic tape. As the ratio of L to W (where L is the length of tape between the entry and exit guides and W is the width of the tape) goes down, the problem of nonuniform tape tension and associated track distortion with changes in guide spacing becomes acute. Typically, the prior art L to W ratios have been in the order of 20 or even 30 to 1. A high L to W ratio helps to eliminate the sensitivity of the track distortion to guide spacing at the penalty of giving up information storage capacity on the tape. The storage capacity of the tape goes down as the width of the tape decreases. When the length to width ratio goes below 10 to 1, the track distortion problem becomes serious and can cause a rotating head to misread data from distorted adjacent tracks crossing the path of the head.
The need for dynamically adjusting guide spacing is due to skew error. Skew error is caused by tendency of magnetic tape to change its physical characteristics under different environmental conditions. It is well known that humidity and temperature affect the magnetic tape to the extent it may change its length and width. These changes are small; however, when dealing with high density tracks written across a wide magnetic tape, they can cause a significant skew error, i.e., lack of parallelism between the tracks on the tape and the path of the rotating head. Therefore, there is a primary need to handle the skew error correction problem by dynamically adjusting the edge guiding of magnetic tape as it helically wraps the mandrel without, at the same time, introducing distortion into the tracks on the tape.